Flame ionization detection in supercritical fluid chromatography is traditionally provided by gas chromatography (GC) instrumentation. Such instrumentation is designed to operate at relatively low mobile phase flow rates. Typical mobile phase flow rates for capillary GC columns are 0.5-15 mL/min, and typical flow rates for packed GC columns are 10-60 mL/min. Analytical-scale packed column SFC generates much higher mobile phase flow rates. Upon decompression, SFC mobile phases can generate mobile phase flow rates up to or greater than 2,500 mL/min. In some applications, the flow is split to reduce the flow rate to the detector. Splitting the flow, however, negatively impacts sensitivity and introduces quantitation variability related to variable split flow values. In other applications, the FID burner diameter can be altered to reduce the linear velocity of the decompressed mobile phase flow through the burner in order to maintain a steady flame. See K. B. Thurbide, S. Gilbert. Canadian Journal of Chemistry 82 (2004) 479-482.
Chromatographic columns are available in a variety of different internal diameters (i.d.). As a result, chromatographic columns have a wide variety of optimum mobile phase flow rates. To accommodate these various rates in an FID while maintaining a steady flame is difficult because the alteration or adjustment of the burner inner diameter is non-trivial. Detector hardware alterations and lengthy optimizations must be performed every time the separation column geometry is changed in a SFC-FID system to ensure optimal detector sensitivity. To provide nominal, or eventually optimized, detector sensitivity and avoid extinguishing the flame, the burner diameter must be matched to a column flow rate to ensure adequate decompressed mobile phase linear velocity.